Plasma treatment is a commonly used process for cleaning, etching, and in general chemically functionalizing a surface. Plasma treatment is used in a great variety of applications, including semiconductor fabrication, treatment of metal foils, treatment of polymer articles, and, in general, treatment of surfaces that need to be cleaned and functionalized for improved adhesion and chemical compatibility with other materials.
The present invention addresses the use of plasma treatment for webs moving at high speeds. Modem manufacturing requires materials such as polymer films, fabrics, paper, and metal sheeting to be processed at speeds of the order of hundreds to several thousand feet per minute. More specifically, polymer films such as polypropylene and polyester are manufactured at speeds in the neighborhood of 1 to 5 m/sec. Such films are also metallized for use in food, electronic, capacitor, and packaging applications at speeds as high as 20 m/sec.
Polymer films are usually functionalized during their manufacture using techniques such as corona treatment (see, e.g., C. M. Chan, Polymer Surface Modification and Characterization, Hanser/Gardner Publications, Inc., Ch. 6, pp. 225-279, Cincinnati (1994); D. M. Brewis, "Factors Affecting the Metallization of Plastics", Electrochemical Society Proceedings, Vol. 97-30, Abstract No. 1652, p. 1914 (1997); A. Ahlbrandt, U.S. Pat. No. 4,774,061, issued Sep. 27, 1988; and S. R. Sabreen, U.S. Pat. No. 5,051,586, issued Sep. 24, 1991) and/or flame treatment (see, e.g., C. M. Chan, supra; W. Osthoff, U.S. Pat. No. 4,627,135, issued Dec. 9, 1986; and W. G. Wise, U.S. Pat. No. 4,610,627, issued Sep. 9, 1986). Such treatment is used to generate polar functional groups on the film surface, which prepares the films for subsequent processes such as printing, coating, laminating and metallizing.
Prior to the metallization process, polymer films--regardless of pretreatment--are plasma-treated in the vacuum chamber to further clean the surface and promote adhesion of the metal to the polymer surface (see, e.g., C. M. Chan, supra; G. L Robertson, supra; Q. T. Le et al., "Oxygen and carbon dioxide plasma modifications of PET surfaces. XPS study on adhesion to aluminum", Electrochemical Society Proceedings, Vol. 97-30, Abstract No. 1660, p. 1922 (1997); and A. Nihlstrand et al., "Plasma Treatment of Polyolefins: Influence of Plasma Parameters and Material Composition on Paint Adhesion", Electrochemical Society Proceedings, Vol. 97-30, Abstract No. 1662, p. 1924 (1997); and H. Uchiyama et al, U.S. Pat. No. 5,543,017, issued Aug. 6, 1996).
Plasma treatment in the past was performed almost exclusively in a vacuum chamber. More recently, there have been several attempts to extend the plasma treatment process to atmospheric pressure, which eliminates costly pumping systems and allows the process to be performed in-line with other operations (see, e.g., T. Tanisaki et al, U.S. Pat. No. 5,391,855, issued Feb. 21, 1995; and A. Yializis et al, "Superior Polymer Webs via in situ Surface Functionalization", Society of Vacuum Coaters, 39th Annual Technical Conference Proceedings, (1996)).
Effective treatment varies from one application to another and it is usually measured in exposure at a given energy level per unit area, or J/cm.sup.2. In batch processes, articles immersed in a plasma environment can easily be treated at a desired level of treatment by varying exposure time. Additional exposure of a few seconds, even minutes, is not out of line with other operations such as fixturing of the articles and pumping. In applications that require treatment of moving webs, higher level of treatment usually dictates higher plasma power and/or long plasma reactors, which can make plasma treatment economically prohibitive. This is especially true for modem aluminum metallizing operations, which are performed at speeds in the range of 10 to 20 m/sec. The object of the present invention is the development of an efficient highly energetic plasma reactor for treating webs moving at high speed.